1. Field of the Invention
This invention relates, generally, to water purifying systems. More particularly, it relates to a low pressure reverse osmosis water purifying system for making and storing product water in a closed pressure vessel in the absence of back pressure.
2. Description of the Prior Art
There are two major categories of reverse osmosis (R.O.) water purifying systems: 1) Those that discharge product water into an open container; and 2) Those that discharge product water into an enclosed container.
The second category has two major subcategories: 1) Systems that discharge product water into an enclosed pressure vessel against back pressure created by an air cell within the vessel; and 2) Systems that discharge product water, in the absence of back pressure, into an enclosed pressure vessel and into a flexible water cell, thereby displacing waste water to drain.
The advantage of open storage systems is that they use all available line pressure to drive water molecules through semi-permeable R.O. membranes, there being no back pressure in such systems. However, costly demand pumps are required to pressurize outgoing product water in such systems. Moreover, such open storage systems are obviously subject to air-borne contamination and are somewhat inconvenient to operate.
Closed container systems of the air cell type are not as subject to air-borne contamination as are open storage systems, but they are subject to the back pressure of the air cells which, essentially, reduces the pressure differential across the R.O. membrane, thereby reducing the quality and quantity of R.O. product water made in a given time. Product water quality particularly suffers if it is frequently drawn off and replaced in small quantities, as typically occurs in household usage.
Moreover, as the air cell-propelled water is emptied from the air cell, the air cell gradually loses pressure and the dispensing flow rate of the product water declines.
While making and storing product water, all R.O. systems waste a predetermined amount of total water to drain; the wasted water is known as "slow flush" water. Some air cell systems continue to waste slow flush water even after the tank has been filled with product water. Accordingly, most air cell systems include an automatic shut-off valve that stops feed water flow, and thus further production of slow flush waste water, when the storage tank is full and reaches 60%-70% of line pressure. This technique, while reducing waste, has the undesireable side effect of also reducing the quantity and quality of the product water and its dispensing flow rate.
The use of demand pumps for increasing dispensing pressure for open storage systems was mentioned above. Slow flow line booster pumps have been used with air cell closed storage systems to overcome low line pressure problems. These are electrically powered with attendant noise, installation, and cost disadvantages. A permeate pump, hydraulically powered by slow flush waste water to drain, slowly raises product water pressure in the air cell tank.
All of these methods provide some improvements, but the open storage--demand pump solution does not overcome airborne contamination and the above-mentioned electrical disadvantages. The line pump almost overcomes the disadvantages of the air cell-created back pressure, but is subject to the above-mentioned disadvantages of electrical equipment. The permeate pump is too slow to practically overcome the deficit of air cell-induced back pressure.
The preferred storage tank is a pressure vessel containing two water-filled compartments of approximately the same size; it is therefore known as a "water-on-water" accumulator. The physical separation between the compartments is movable or flexible so that water pressure in a first compartment influences the water pressure in the second compartment. Each compartment is accessed by different fluid sources so that one compartment may be filling while the other one is emptying. Thus, little or no pressure drop occurs across the compartments until the wall of the compartment acts as a valve and blocks the exit port of the emptying compartment.
Both compartments are pressurized, when product water is drawn, by a control valve having multiple sensing and valving capabilities. Both compartments are then depressurized by the control valve when product water is filling one compartment and displacing water from the other compartment to drain. Limited drain flow (slow flush) is shut off by the control valve when the product water compartment is filled. Valving action is started by manual opening and closing of a product water faucet or refrigerator outlet.
U.S. Pat. No. 4,176,063 to Tyler discloses a hydraulic powered control valve, of the diaphragm type, for a water cell system, and U.S. Pat. No. 4,705,625 to the present inventor discloses a hydraulic control valve, of the piston type, for a water cell system. At lower feed water pressures, diaphragm type control valves open and close more slowly than do piston type control valves. The speed of a diaphragm type valve is in direct proportion to feed water pressure. A piston type control valve maintains a constant, faster opening and closing rate across a wider range of feed water pressures, i.e., such valves are not as dependent upon feed water pressures. The speed of opening and closing valves is an important aspect of an R.O. water purifying system because it affects the operation of collateral equipment in the system.
A piston type control valve thus operates more dependably than a diaphragm type control valve at low feed water pressures, it provides a faster and more complete valving action, and it provides faster product and flush flow rates at any given line water pressure.
Further examples of prior art systems are found in U.S. Pat. Nos. 3,493,496, 3,568,843, 3,542,199, 3,726,793, 3,887,463, 4,971,689, 5,254,243, and 5,460,716.
The most relevant prior art to the present disclosure is believed to be U. S. Pat. No. 4,705,625 to the present inventor. Although the piston type control valve disclosed in that patent performs in a superior fashion to diaphragm type valves, there remains a need to widen the operating range of feed water pressures and to increase the product water and flush flow rate.
There is also a need for an easily interchangeable, self-cleaning brine restrictor, and for a more positive automatic shut-off. Ideally, the brine restrictor and the shut-off means should be built into the control valve as an integral part thereof.
One drawback of the present inventor's earlier spring-biased piston control valve is that it requires a relatively high pressure relative to line water pressure to move the valve for two essential functions: the automatic turning on and off of product water. Both functions cause product water pressure to increase to nearly line pressure and that causes a very low pressure differential across the R.O. membrane. Product pressure increase and subsequent valving action could even stop if the total dissolved solids in the feed water were to exceed a certain threshhold.
Accordingly, there is a need for a piston type control valve that can operate effectively on lower water pressure to move the piston against full line pressure without the aid of a strong mechanical bias means, such as a spring, which bias means would ultimately limit fast flow at the faucet when the piston is moving against the bias means.
An automatic self-cleaning brine restrictor valve which is integral to the control valve is also needed.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the needed improvements could be provided.